The present invention relates to a disposable unit for filtering lubricating oil in internal combustion engines, and in particular in diesel engines. For filtering lubricating oil in such engines, disposable filter units are known consisting of a cup-shaped container containing one or more filter cartridges, which is arranged to be screwed directly onto the engine block to receive the oil to be filtered from the lubricating oil pump and feed it to the various lubrication points provided in the engine.
The most simple filters of this type are the so-called full-flow filters, i.e. filters containing a single oil cartridge which is traversed by the whole throughput made available by the lubricating oil pump, and which feeds all the filtered oil to the lubricating points provided in the engine. These filters have operational limitations derived from their intrinsic method of operation. In this respect, the filtering efficiency of said filters depends on the filter area and on the porosity of the filter material, whereas their life expectancy is inversely proportional to this porosity.
Thus to obtain a filter having an acceptable life, large-surface medium-porosity filter cartridges must be used but these have the defect of a low initial filtering efficiency which increases proportionally with time until they are completely clogged. On the other hand, reducing the filter material porosity leads to a shortening of the filter life and requires a too high lubricating oil pump power, whereas increasing the porosity on the one hand increases the filter life by delaying its complete clogging but on the other hand lowers the filtering efficiency during the initial operating period of the filter.
To obviate these drawbacks, which are obviously the result of a compromise between the requirement of engine protection on the one hand and economy and small overall size of the filter unit on the other hand, so-called double filtration units have been used. These latter filters, which are also known as mixed-circuit filters, provide a better filtration than that obtainable with the aforesaid full-flow filters. They consist of a casing housing two filter cartridges arranged in parallel to receive the oil to be filtered from one and the same compartment, but feed the filtered oil to two different destinations. Specifically, the larger filter cartridge, which is also that of coarser porosity, feeds the filtered oil directly to the engine users, whereas the smaller filter cartridge of finer porosity, used for so-called fine filtering, feeds the oil directly into the engine sump through separate ducts. Thus most of the oil passes through the coarser porosity filter cartridge to be fed to the lubrication points provided in the engine, whereas a smaller fraction of the oil is recycled to the sump with all impurities completely removed. Statistically, this continuous mixing in the engine sump of a small throughput of highly filtered oil with a larger throughput of normally filtered oil greatly increases the average degree of oil filtration.
This type of filter, known as a mixed-circuit filter, has however the serious drawback that it can be fitted only to engines designed for this purpose. In this respect, at the point where the filter unit is screwed on, there must be a duct provided for recirculation directly into the sump, a feed duct to the various lubrication points and a duct for oil feed to the filter. These filters cannot therefore be applied to engines which do not possess a circuit for recirculating part of the oil directly to the engine sump. To obtain the same filtering efficiency without too much complication in engines not designed for mixed-circuit filters, an attempt has been made to fit into one and the same filter units two cartridges of different porosity operating in parallel and feeding into the same lubrication circuit feed duct. However, this attempt has proved ineffective because the filter cartridge of finer porosity is the first to become clogged and after a certain period of operation becomes completely inefficient, making the entire unit ineffective.
The object of the present invention to provide a filter unit which combines the efficiency of mixed-circuit filter units with the simplicity of application of full-flow filter units, and which can be applied both to engines designed to receive full-flow filter units and to engines designed to receive mixed-circuit filter units.
This is attained by a filter unit using two filter cartridges of different porosity operating in parallel, of which at least one, that of the smaller filtering area and finer porosity, is arranged in series with and downstream of a third filter cartridge of coarser porosity.